A sample shut off valve is a common need in the gas chromatograph. The purpose of the sample shut off valve is to temporarily block the flow of the sample into the sample valve of the gas chromatograph. This allows the volume of vapor in the sample loop attached to the sample valve to equilibrate to atmospheric pressure prior to being injected into the chromatographic column when the sample valve is actuated. This reduces the influence of sample pressure on the quantified analytical results.
An implementation of the sample shut off valve function is to add a separate valve in the chromatograph oven. This valve is connected in series with the sample flow but is upstream of the sample valve. The sample shut off valve is then activated to block the flow for a period of a few seconds. The time is typically chosen to allow adequate time for the sample loop and all associated tubing paths downstream of the sample shutoff valve to equilibrate to atmospheric pressure.
The addition of the separate valve adds product costs due to the material cost of the valve, the labor for additional plumbing, and also consumes space in the gas chromatograph oven compartment.
Some valves available to the industry have an internal means of blocking the sample flow path prior to blocking the rest of the flow path prior to blocking the rest of the flow paths by having the parts situated such that the sample inlet port physically closes first when the valve mechanism is switched (typically a rotary or slider valve). Since the valve mechanism is directly coupled to the rest of the switching mechanism, the sample inlet flow is only blocked for a very short duration (<100 milliseconds). This does not allow adequate time to equilibrate the sample loop volume to atmospheric pressure.
Therefore, there is a significant need for a sample valve with an integrated sample shut off function.